Brushless DC electric motor


Brushless DC electric motor
A microprocessor-controlled BLDC motor powering a micro remote-controlled airplane. This external rotor motor weighs 5 grams, consumes approximately 11 watts (15 millihorsepower) and produces thrust of more than twice the weight of the plane.

Contents

Brushless DC motors (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are electric motors powered by direct-current (DC) electricity and having electronic commutation systems, rather than mechanical commutators and brushes. The current-to-torque and frequency-to-speed relationships of BLDC motors are linear.

BLDC motors may be described as stepper motors, with fixed permanent magnets and possibly more poles on the rotor than the stator, or reluctance motors. The latter may be without permanent magnets, just poles that are induced on the rotor then pulled into alignment by timed stator windings. However, the term stepper motor tends to be used for motors that are designed specifically to be operated in a mode where they are frequently stopped with the rotor in a defined angular position; this page describes more general BLDC motor principles, though there is overlap.

Brushless versus Brushed motor

Brushed DC motors have been in commercial use since 1886.[1][2] BLDC motors, however, have only been commercially possible since 1962.[3][4]

Limitations of brushed DC motors overcome by BLDC motors include lower efficiency and susceptibility of the commutator assembly to mechanical wear and consequent need for servicing, at the cost of potentially less rugged and more complex and expensive control electronics. BLDC motors develop maximum torque when stationary[citation needed] and have linearly decreasing torque[citation needed] with increasing speed as shown in the adjacent figure.

Brushless DC Electric Motor Torque-Speed Characteristics

A BLDC motor has permanent magnets which rotate and a fixed armature, eliminating the problems of connecting current to the moving armature. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system.

The interface circuitry between a digital controller and motor. The waveforms show multiple transitions between high and low voltage levels, approximations to a trapezoid or sinusoid which reduce harmonic losses. The circuit compensates for the induction of the windings, regulates power and monitors temperature.

BLDC motors offer several advantages over brushed DC motors, including more torque per weight, more torque per watt (increased efficiency), increased reliability, reduced noise, longer lifetime (no brush and commutator erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the windings are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter.

The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can weaken the magnets. (Magnets demagnetize at high temperatures, the Curie point, and for neodymium-iron-boron magnets this temperature is lower than for other types.) A BLDC motor's main disadvantage is higher cost, which arises from two issues. First, BLDC motors require complex electronic speed controllers to run. Brushed DC motors can be regulated by a comparatively simple controller, such as a rheostat (variable resistor). However, this reduces efficiency because power is wasted in the rheostat. Second, some practical uses have not been well developed in the commercial sector. For example, in the Radio Control (RC) hobby arena, brushless motors are often hand-wound while brushed motors are usually machine-wound. (Nevertheless, see "Applications", below.)

BLDC motors are often more efficient at converting electricity into mechanical power than brushed DC motors. This improvement is largely due to the absence of electrical and friction losses due to brushes. The enhanced efficiency is greatest in the no-load and low-load region of the motor's performance curve. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency.

AC induction motors require induction of magnetic field in the rotor by the rotating field of the stator; this results in the magnetic and electric fields being out of phase. The phase difference requires greater current and current losses to achieve power. BLDC motors are microprocessor-controlled to keep the stator current in phase with the permanent magnets of the rotor, requiring less current for the same effect and therefore resulting in greater efficiency.

In general, manufacturers use brush-type DC motors when low system cost is a priority but brushless motors to fulfill requirements such as maintenance-free operation, high speeds, and operation in explosive environments where sparking could be hazardous.

Controller implementations

Because the controller must direct the rotor rotation, the controller requires some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called sensorless controllers. Like an AC motor, the voltage on the undriven coils is sinusoidal, but over an entire commutation the output appears trapezoidal because of the DC output of the controller.

The controller contains 3 bi-directional outputs to drive high-current DC power, which are controlled by a logic circuit. Simple controllers employ comparators to determine when the output phase should be advanced, while more advanced controllers employ a microcontroller to manage acceleration, control speed and fine-tune efficiency.

Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run briefly backwards, adding even more complexity to the startup sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position.

The controller unit is often referred to as an "ESC", meaning Electronic Speed Controller.

Variations in construction

The four poles on the stator of a two-phase BLDC motor. This is part of a computer cooling fan; the rotor has been removed.
Schematic for delta and wye winding styles. (This image does not illustrate the motor's inductive and generator-like properties)

BLDC motors can be constructed in several different physical configurations: In the 'conventional' (also known as 'inrunner') configuration, the permanent magnets are part of the rotor. Three stator windings surround the rotor. In the 'outrunner' (or external-rotor) configuration, the radial-relationship between the coils and magnets is reversed; the stator coils form the center (core) of the motor, while the permanent magnets spin within an overhanging rotor which surrounds the core. The flat type, used where there are space or shape limitations, uses stator and rotor plates, mounted face to face. Outrunners typically have more poles, set up in triplets to maintain the three groups of windings, and have a higher torque at low RPMs. In all BLDC motors, the coils are stationary.

There are also two electrical configurations having to do with how the wires from the windings are connected to each other (not their physical shape or location). The delta configuration connects the three windings to each other (series circuits) in a triangle-like circuit, and power is applied at each of the connections. The wye ("Y"-shaped) configuration, sometimes called a star winding, connects all of the windings to a central point (parallel circuits) and power is applied to the remaining end of each winding.

A motor with windings in delta configuration gives low torque at low rpm, but can give higher top rpm. Wye configuration gives high torque at low rpm, but not as high top rpm.[5]

Although efficiency is greatly affected by the motor's construction, the wye winding is normally more efficient. In delta-connected windings, half voltage is applied across the windings adjacent to the undriven lead (compared to the winding directly between the driven leads), increasing resistive losses. In addition, windings can allow high-frequency parasitic electrical currents to circulate entirely within the motor. A wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses.

From a controller standpoint, the two styles of windings are treated exactly the same, although some less expensive controllers are designed to read voltage from the common center of the wye winding.

Spindle motor from a 3.5" floppy disk drive. The coils are copper wire coated with green film insulation. The rotor (upper right) has been removed and turned upside-down. The gray ring just inside its cup is a multi-pole permanent magnet.

See Also:Programmable Magnets

AC and DC power supplies

It's helpful to consider three types of motors:

  • Direct current (DC) motor: DC applied to both the stator and the rotor (via brushes and commutator), or else a permanent magnet stator. A BLDC motor has switched DC fed to the stator, and a permanent magnet rotor.
  • Synchronous (or stepping) motor (AC): AC in one, DC in the other (i.e., rotor or stator). If it has a permanent-magnet rotor, it is much like a BLDC motor.
  • Induction motor (AC): AC in both stator and rotor (mentioned for completeness).

Although BLDC motors are practically identical to permanent magnet AC motors, the controller implementation is what makes them DC. While AC motors feed sinusoidal current simultaneously to each of the legs (with an equal phase distribution), DC controllers only approximate this by feeding full positive and negative voltage to two of the legs at a time. The major advantage of this is that both the logic controllers and battery power sources also operate on DC, such as in computers and electric cars. In addition, the approximated sine wave leaves one leg undriven at all times, allowing for back-EMF-based sensorless feedback.

Vector drives are DC controllers that take the extra step of converting back to AC for the motor; they are sophisticated inverters. The DC-to-AC conversion circuitry is usually expensive and less efficient, but these have the advantage of being able to run smoothly at very low speeds or completely stop (and provide torque) in a position not directly aligned with a pole. Motors used with a vector drive are typically called AC motors. When running at low speeds and under load, they don't cool themselves significantly; temperature rise has to be allowed for.

A motor can be optimized for AC (i.e. vector control) or it can be optimized for DC (i.e. block commutation). A motor which is optimized for block commutation will typically generate trapezoidal EMF. One can easily observe the shape of the EMF by connecting the motor wires (at least two of them) to an oscilloscope and then hand-cranking/spinning the shaft.

Another very important issue, at least for some applications like automotive vehicles, is the constant power speed ratio of a motor (CPSR). The CPSR has direct impact on needed size of the inverter. Example: A motor with a high CPSR in a vehicle can deliver the desired power (e.g. 40 kW) from 3,000 rpm to 12,000 rpm, while using a 100 A inverter. A motor with low CPSR would need a 400 A inverter in order to do the same.

Stepping motors can also operate as AC synchronous motors (for instance, the Slo-Syn by Superior Electric), or the unusual battery-powered quartz-timed micropower clock that has a continuous-motion sweep second hand.

KM rating

"KM" is the motor constant, it can be measured in Nm torque per square root of W resistive loss. KM is sometimes called the "motor size constant". The motor constant is winding independent (as long as the same conductive material used for wires); e.g. winding a motor with 6 turns with 2 parallel wires instead of 12 turns single wire will double the Kv velocity constant, but the KM remains unchanged.

KM can be used for selecting the size of a motor to use in an application. Kv can be used for selecting the winding to use in the motor.

Kv rating

"Kv" is the motor velocity constant, measured in RPM per volt (not to be confused with "kV," the abbreviation for "kilovolt")[6] . The Kv rating of a brushless motor is the ratio of the motor's unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the "back-EMF"). For example, a 5,700 Kv motor, supplied with 11.1 V, will run at a nominal 63,270 rpm (=5700 * 11.1).

By Lenz's law, a running motor will create a back-EMF proportional to the RPM. Once a motor is spinning so fast that the back-EMF is equal to the battery voltage (also called DC line voltage), it is impossible for the ESCs to "speed up" that motor, even with no load.

Applications

BLDC motors fulfill many functions originally performed by brushed DC motors, but cost and control complexity prevents BLDC motors from replacing brushed motors completely in the lowest-cost areas. Nevertheless, DC motors have come to dominate many applications, particularly devices such as computer hard drives and CD/DVD players. Small cooling fans in electronic equipment are powered exclusively by BLDC motors. They can be found in cordless power tools where the increased efficiency of the motor leads to longer periods of use before the battery needs to be charged. Low speed, low power BLDC motors are used in direct-drive turntables for "analog" audio discs.

Transport

High power BLDC motors are found in electric vehicles and hybrid vehicles. These motors are essentially AC synchronous motors with permanent magnet rotors.

The Segway Scooter and Vectrix Maxi-Scooter use BLDC technology.

A number of electric bicycles use BLDC motors that are sometimes built into the wheel hub itself, with the stator fixed solidly to the axle and the magnets attached to and rotating with the wheel. The bicycle wheel hub is the motor. This type of electric bicycle also has a standard bicycle transmission with pedals, sprockets, and chain that can be pedaled along with, or without, the use of the motor as need arises.[7]

Heating and ventilation

There is a trend in the HVAC and refrigeration industries to use BLDC motors instead of various types of AC motors. The most significant reason to switch to a BLDC motor is the dramatic reduction in power required to operate them versus a typical AC motor.[8][9] While shaded-pole and permanent split capacitor motors once dominated as the fan motor of choice, many fans are now run using a BLDC motor. Some fans use BLDC motors also in order to increase overall system efficiency.[10]

In addition to the BLDC motor's higher efficiency, certain HVAC systems (especially those featuring variable-speed and/or load modulation) use BLDC motors because the built-in microprocessor allows for programmability, better control over airflow, and serial communication.

Industrial engineering

See Stepper motor, Servo motor.

Stepper motor

The stepper motor is used in microprocessor and microcontroller-based and robotic equipment, as it consumes less power and provides accurate movement of robotic arms. Semiconductor producers include Infineon Technologies, Texas Instruments and Microchip. Infineon offers so-called LIN stepper motors used in applications such as instrumentation and gauges, CNC machining, multi-axis positioning, printers and surveillance equipment.[11]

Model engineering

BLDC motors are a popular motor choice for model aircraft including helicopters. Their favorable power-to-weight ratios and large range of available sizes, from under 5 gram to large motors rated at thousands of watts, have revolutionized the market for electric-powered model flight.

Their introduction has redefined performance in electric model aircraft and helicopters, displacing virtually all brushed electric motors. They have also encouraged a growth of simple, lightweight electric model aircraft, rather than the previous internal combustion engines powering larger and heavier models. The large power-to-weight ratio of modern batteries and brushless motors allows models to ascend vertically, rather than climb gradually. The low noise and lack of mess compared to small glow fuel internal combustion engines that are used is another reason for their popularity.

Legal restrictions for the use of combustion engine driven model aircraft in some countries have also supported the shift to high-power electric systems.

Their popularity has also risen in the Radio Controlled Car, Buggy, and Truck scene, where sensor-type motors (with an extra six wires, connected to Hall effect sensors) allow the position of the rotor magnet to be detected. Brushless motors have been legal in RC Car Racing in accordance to ROAR (the American governing body for RC Car Racing), since 2006. Several RC Car Brushless motors, feature replaceable and upgradeable parts, such as sintered neodymium-iron-boron (rare earth magnets), ceramic bearings, and replaceable motor timing assemblies. These motors as a result are quickly rising to be the preferred motor type for electric on and off-road RC racers and recreational drivers alike, for their low maintenance, high running reliability and power efficiency (most Sensored motors have an efficiency rating of 80% or greater).

See also

References

External links


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